The theme of central modality segregation and electrophysiological characterization of quantitative properties in medullary and thalamic somatosensory neurons is to be extended and amplified by anatomical studies of tracing projection neurons by using the method employing retrograde axonal transport of horseradish peroxidase (HRP). Electrophysiological mapping of specific mechanoreceptor representation in the thalamic ventrobasal complex will be performed in cats and monkeys using a chronic closed-skull preparation. Examples of each receptor category will be subjected to extensive quantitative study of input-output functions. Cluster of modality-specific neurons will be iontophoretically labelled with HRP from the recording micropipette for identifying the origin of lemniscal and spinal cord projection neurons and hopefully will distinguish the projection of specific nociceptors and sensitive mechanoreceptors. Emphasis will be placed on mapping suspected VL sites for spinothalamic termination. Continuing studies of the sensory trigeminal nuclear complex will include a laminar analysis of nucleus caudalis by focal HRP labelling, as well as applying HRP to peripheral afferent receptors and root fibers. The cortical projection of VB will be further studied by injecting HRP into the sub-fields of somatic cortex known to receive specific afferent input from muscle, rapidly and slowly adapting mechanoreceptors and selected sectors of areas 2 and 1 in order to examine the thalamic pattern of modality segregation anatomically. These studies should provide an anatomical basis for comparing the properties of neurons of anterolateral and lemniscal systems and relating these data to sensory discrimination in man.